A multicrystalline silicon substrate has been widely used as a solar cell substrate. The multicrystalline silicon substrate is obtained by cutting a multicrystalline silicon ingot (Japanese Patent Application Laid-Open No. 11-288881, etc.).
One of the processes for producing the multicrystalline silicon ingot comprises holding a silicon powder as a source material in a crucible, heating the silicon powder by a heater surrounding the crucible to melt the silicon powder, slowly moving the crucible downward away from the heater to cool the crucible from the lower part to obtain the multicrystalline silicon ingot (Japanese Patent Publication No. 4-68276, etc.). The multicrystalline silicon ingot obtained by the process has individual crystal grains of an elongated shape arranged substantially in one direction. Such a multicrystalline silicon ingot is generally called a multicrystalline silicon ingot made by directional solidification.
Generally, a substrate is obtained from an ingot by cutting with a wire saw. In the case of a conventional solar cell multicrystalline silicon substrate, the multicrystalline silicon ingot made by directional solidification is cut orthogonal to the longitudinal direction of crystal grains (Japanese Patent Publication No. 4-68276, Japanese Patent Application Laid-Open No. 2000-1308, etc.). Namely, crystal grains are cut so that the longitudinal direction thereof is in agreement with a normal line of a principal surface of a substrate. This cutting method is herein referred to as “transverse cutting.”
The “transverse cutting” is selected as a conventional cutting method of an ingot because if a number of grain boundaries exist approximately perpendicularly to a thickness direction of a substrate, charge transfer is prevented at the grain boundary portions to cause charge recombination and loss in electric current. In other words, the agreement of the longitudinal direction of the crystal grains with the thickness direction of the cut substrate decreases grain boundaries crossing the thickness direction of the substrate and can suppress the loss in current, so that the transverse cutting has hitherto been adopted.
In addition, Japanese Patent Application Laid-Open No. 10-98205 describes a technology in which instead of making a solar cell in the above mentioned multicrystalline silicon substrate itself, a silicon film with a low impurity content is grown on an inexpensive metallurgical grade multicrystalline silicon substrate having a high impurity concentration, and a solar cell is made in the thus grown silicon film.